Add dynamic bindings and better error positions

AST.cs

@@ -1,4 +1,3 @@
-using System.CodeDom.Compiler;
 using System.Collections.Generic;
 
 namespace Finn.AST;

Expr.g.cs

@@ -7,7 +7,7 @@
 
 namespace Finn.AST;
 
-public abstract record Expr() {
+public abstract record Expr(Token Start) {
 	public abstract TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor);
 }
 public interface IExprVisitor<TContext, TResult> {
@@ -27,105 +27,105 @@ public interface IExprVisitor<TContext, TResult> {
 	TResult visitLetExpr(TContext context, Let expr);
 	TResult visitWhenExpr(TContext context, When expr);
 }
-public partial record Sequence(Expr Left, Expr Right) : Expr()
+public partial record Sequence(Expr Left, Expr Right) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitSequenceExpr(context, this);
 	}
 }
-public partial record Binary(Expr Left, Token Op, Expr Right) : Expr()
+public partial record Binary(Expr Left, Token Op, Expr Right) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitBinaryExpr(context, this);
 	}
 }
-public partial record Grouping(Expr Expression) : Expr()
+public partial record Grouping(Expr Expression, Token LParen) : Expr(LParen)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitGroupingExpr(context, this);
 	}
 }
-public partial record Literal(System.Object Value) : Expr()
+public partial record Literal(System.Object Value, Token Token) : Expr(Token)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitLiteralExpr(context, this);
 	}
 }
-public partial record Unary(Token Op, Expr Right) : Expr()
+public partial record Unary(Token Op, Expr Right) : Expr(Op)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitUnaryExpr(context, this);
 	}
 }
-public partial record If(Expr Condition, Expr Then, Expr Else) : Expr()
+public partial record If(Expr Condition, Expr Then, Expr Else, Token IfToken) : Expr(IfToken)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitIfExpr(context, this);
 	}
 }
-public partial record Variable(Token Value) : Expr()
+public partial record Variable(Token Value) : Expr(Value)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitVariableExpr(context, this);
 	}
 }
-public partial record List(Expr[] Elements) : Expr()
+public partial record List(Expr[] Elements, Token LBracket) : Expr(LBracket)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitListExpr(context, this);
 	}
 }
-public partial record Variant(Token Tag, Expr? Argument) : Expr()
+public partial record Variant(Token Tag, Expr? Argument, Token Backtick) : Expr(Backtick)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitVariantExpr(context, this);
 	}
 }
-public partial record Record(Field[] Extensions, BaseRecord? Base) : Expr()
+public partial record Record(Field[] Extensions, BaseRecord? Base, Token LBrace) : Expr(LBrace)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitRecordExpr(context, this);
 	}
 }
-public partial record Selector(Expr Left, Token FieldName) : Expr()
+public partial record Selector(Expr Left, Token FieldName) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitSelectorExpr(context, this);
 	}
 }
-public partial record Indexer(Expr Left, Expr Index) : Expr()
+public partial record Indexer(Expr Left, Expr Index) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitIndexerExpr(context, this);
 	}
 }
-public partial record Call(Expr Left, Expr?[] Arguments) : Expr()
+public partial record Call(Expr Left, Expr?[] Arguments) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitCallExpr(context, this);
 	}
 }
-public partial record Let(Binding[] Bindings, Expr Body) : Expr()
+public partial record Let(Binding[] Bindings, Expr Body, Token LetToken) : Expr(LetToken)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitLetExpr(context, this);
 	}
 }
-public partial record When(Expr Head, VarBinding[] Cases) : Expr()
+public partial record When(Expr Head, VarBinding[] Cases, Token WhenToken) : Expr(WhenToken)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{

Interpreter.cs

@@ -86,7 +86,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 	{
 		public ValueTuple visitFieldPatternPattern((object, Env) context, AST.FieldPattern pattern)
 		{
-			return (pattern.Pattern ?? new AST.SimplePattern(pattern.Name)).accept(context, this);
+			return (pattern.Pattern ?? new AST.SimplePattern(pattern.Name, pattern.Name)).accept(context, this);
 		}
 
 		public ValueTuple visitRecordPatternPattern((object, Env) context, AST.RecordPattern pattern)
@@ -410,8 +410,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 	public object visitIfExpr(Env env, AST.If expr)
 	{
 		var cond = evaluate(env, expr.Condition);
-		// TODO Maybe I should token info in the AST.
+		var vb = checkBoolOperand(expr.Condition.Start, cond);
-		var vb = checkBoolOperand(new Token(TokenType.If, "if", null, new(0, 1, 1)), cond);
 		if (vb == Variant.True)
 		{
 			return evaluate(env, expr.Then);
@@ -424,10 +423,8 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitIndexerExpr(Env env, AST.Indexer expr)
 	{
-		// TODO use real token
+		var left = checkListOperand(expr.Left.Start, evaluate(env, expr.Left));
-		var tok = new Token(TokenType.LBracket, "[", null, new(0, 1, 1));
+		var index = checkNumberOperand(expr.Index.Start, evaluate(env, expr.Index));
-		var left = checkListOperand(tok, evaluate(env, expr.Left));
-		var index = checkNumberOperand(tok, evaluate(env, expr.Index));
 		try
 		{
 			var item = left[index];
@@ -452,7 +449,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 					catch (Exception e)
 					{
 						// TODO use real info
-						var tok = new Token(TokenType.Let, "let", null, new(0, 1, 1));
+						var tok = new Token(TokenType.Let, "let", null, new(0, 1, 1), new(0, 1, 1));
 						throw new RuntimeError(tok, e.Message);
 					}
 					break;
@@ -465,8 +462,6 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitListExpr(Env env, AST.List expr)
 	{
-		// TODO use real token
-		var tok = new Token(TokenType.LBracket, "[", null, new(0, 1, 1));
 		List l = List.Empty;
 		foreach (var itemExpr in expr.Elements)
 		{
@@ -474,7 +469,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			if (!l.IsEmpty)
 			{
 				try { checkTypesEqual(l[0], item); }
-				catch { throw new RuntimeError(tok, "List items must all have same type."); }
+				catch { throw new RuntimeError(itemExpr.Start, "List items must all have same type."); }
 			}
 			l = l.Add(item);
 		}
@@ -488,15 +483,13 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitRecordExpr(Env env, AST.Record expr)
 	{
-		// TODO use real token
-		Token tok = new Token(TokenType.LBrace, "{", null, new(0, 1, 1));
 		Record rec = Record.Empty;
 		if (expr.Base != null)
 		{
 			var baseRecValue = evaluate(env, expr.Base.Value);
 			if (baseRecValue is not Record)
 			{
-				throw new RuntimeError(tok, "Base record must be a record.");
+				throw new RuntimeError(expr.Base.Value.Start, "Base record must be a record.");
 			}
 			var baseRec = (Record)baseRecValue;
 
@@ -507,17 +500,20 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 				var label = (string)update.Name.Literal!;
 				if (updateLabels.Contains(label))
 				{
-					throw new RuntimeError(tok, "Record updates must be to unique fields.");
+					throw new RuntimeError(update.Name, "Record updates must be to unique fields.");
 				}
 				updateLabels.Add(label);
-				var updateValue = update.Value == null ? env[update.Name] : evaluate(env, update.Value);
+				(var updateValue, var updateValueToken) =
+					update.Value == null ?
+						(env[update.Name], update.Name) :
+						(evaluate(env, update.Value), update.Value.Start);
 				try
 				{
 					baseRec = baseRec.Update(label, updateValue);
 				}
 				catch
 				{
-					throw new RuntimeError(tok, "Field update must have same type as previous value.");
+					throw new RuntimeError(updateValueToken, "Field update must have same type as previous value.");
 				}
 			}
 			rec = baseRec;
@@ -530,7 +526,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			var label = (string)extension.Name.Literal!;
 			if (extLabels.Contains(label))
 			{
-				throw new RuntimeError(tok, "Record extensions must have unique field names.");
+				throw new RuntimeError(extension.Name, "Record extensions must have unique field names.");
 
 			}
 			extLabels.Add(label);
@@ -544,16 +540,14 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 	public object visitSelectorExpr(Env env, AST.Selector expr)
 	{
 		var left = evaluate(env, expr.Left);
-		// TODO Use real token.
+		var r = checkRecordOperand(expr.Left.Start, left);
-		var tok = new Token(TokenType.Period, ".", null, new(0, 1, 1));
-		var r = checkRecordOperand(tok, left);
 		try
 		{
 			return r.Get((string)expr.FieldName.Literal!);
 		}
 		catch
 		{
-			throw new RuntimeError(tok, "Operand must have selected field.");
+			throw new RuntimeError(expr.FieldName, "Operand must have selected field.");
 		}
 	}
 
@@ -600,7 +594,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 	{
 		var head = evaluate(env, expr.Head);
 		// TODO use real info
-		var tok = new Token(TokenType.When, "when", null, new(0, 1, 1));
+		var tok = new Token(TokenType.When, "when", null, new(0, 1, 1), new(0, 1, 1));
 		foreach (var c in expr.Cases)
 		{
 			try

Parser.cs

@@ -158,6 +158,7 @@ class Parser
 		{
 			return null;
 		}
+		var lbrace = previous();
 
 		List<FieldPattern> fields = new List<FieldPattern>();
 		while (!check(TokenType.RBrace, TokenType.Pipe))
@@ -172,7 +173,7 @@ class Parser
 		}
 		var restPattern = match(TokenType.Pipe) ? simplePattern() : null;
 		consume("Expect '}' at end of record pattern.", TokenType.RBrace);
-		return new(fields.ToArray(), restPattern);
+		return new(fields.ToArray(), restPattern, lbrace);
 	}
 
 	private VariantPattern? variantPattern()
@@ -181,24 +182,25 @@ class Parser
 		{
 			return null;
 		}
+		var backtick = previous();
 		var tag = consume("Expect identifier as tag name.", TokenType.Identifier, TokenType.QuotedIdentifier);
 		if (!match(TokenType.LParen))
 		{
-			return new(tag, null);
+			return new(tag, null, backtick);
 		}
 		Pattern argument = pattern();
 		consume("Expect ')' after variant argument.", TokenType.RParen);
-		return new(tag, argument);
+		return new(tag, argument, backtick);
 	}
 
 	private SimplePattern? simplePattern()
 	{
 		if (match(TokenType.Blank))
 		{
-			return new SimplePattern(null);
+			return new SimplePattern(null, previous());
 		}
 
-		return match(TokenType.Identifier, TokenType.QuotedIdentifier) ? new(previous()) : null;
+		return match(TokenType.Identifier, TokenType.QuotedIdentifier) ? new(previous(), previous()) : null;
 	}
 
 	private Pattern pattern()
@@ -233,6 +235,7 @@ class Parser
 		{
 			return ifExpr();
 		}
+		var letToken = previous();
 
 		List<Binding> bindings = new List<Binding>();
 		bindings.Add(parseBinding());
@@ -242,7 +245,7 @@ class Parser
 		}
 		consume("Expect 'in' after let-bindings.", TokenType.In);
 		Expr body = expression();
-		return new Let(bindings.ToArray(), body);
+		return new Let(bindings.ToArray(), body, letToken);
 
 		Binding parseBinding()
 		{
@@ -277,12 +280,13 @@ class Parser
 		{
 			return when();
 		}
+		var ifToken = previous();
 		Expr condition = expression();
 		consume("Expect 'then' after condition.", TokenType.Then);
 		Expr thenCase = expression();
 		consume("Expect 'else' after 'then' case.", TokenType.Else);
 		Expr elseCase = expression();
-		return new If(condition, thenCase, elseCase);
+		return new If(condition, thenCase, elseCase, ifToken);
 	}
 
 	private Expr when()
@@ -291,6 +295,7 @@ class Parser
 		{
 			return primary();
 		}
+		var whenToken = previous();
 		Expr head = expression();
 		consume("Expect 'is' after expression.", TokenType.Is);
 
@@ -300,7 +305,7 @@ class Parser
 		{
 			cases.Add(parseCase());
 		}
-		return new When(head, cases.ToArray());
+		return new When(head, cases.ToArray(), whenToken);
 
 		VarBinding parseCase()
 		{
@@ -357,8 +362,9 @@ class Parser
 	{
 		if (match(TokenType.Number, TokenType.String))
 		{
-			object literal = previous().Literal!;
+			var token = previous();
-			return new Literal(previous().Literal!);
+			object literal = token.Literal!;
+			return new Literal(previous().Literal!, token);
 		}
 
 		if (match(TokenType.Identifier, TokenType.QuotedIdentifier))
@@ -368,9 +374,10 @@ class Parser
 
 		if (match(TokenType.LParen))
 		{
+			var lparen = previous();
 			Expr groupedExpr = expression();
 			consume("Expect ')' after expression.", TokenType.RParen);
-			return new Grouping(groupedExpr);
+			return new Grouping(groupedExpr, lparen);
 		}
 
 		Expr? expr;
@@ -396,6 +403,7 @@ class Parser
 		{
 			return null;
 		}
+		var lbracket = previous();
 
 		List<Expr> elements = new List<Expr>();
 
@@ -421,7 +429,7 @@ class Parser
 				}
 			}
 		}
-		return new Finn.AST.List(elements.ToArray());
+		return new List(elements.ToArray(), lbracket);
 	}
 
 	private Expr? variant()
@@ -430,6 +438,7 @@ class Parser
 		{
 			return null;
 		}
+		var backtick = previous();
 		var tag = consume("Expect identifier as tag name.", TokenType.QuotedIdentifier, TokenType.Identifier);
 		Expr? argument = null;
 		if (match(TokenType.LParen))
@@ -440,7 +449,7 @@ class Parser
 				consume("Expect ')' after variant argument.", TokenType.RParen);
 			}
 		}
-		return new Variant(tag, argument);
+		return new Variant(tag, argument, backtick);
 	}
 
 	private Expr? record()
@@ -449,6 +458,7 @@ class Parser
 		{
 			return null;
 		}
+		var lbrace = previous();
 
 		var extensions = parseFields(TokenType.RBrace, TokenType.Pipe);
 		BaseRecord? baseRecord = null;
@@ -461,7 +471,7 @@ class Parser
 		}
 
 		consume("Expect '}' at end of record literal.", TokenType.RBrace);
-		return new Record(extensions, baseRecord);
+		return new Record(extensions, baseRecord, lbrace);
 
 		Field[] parseFields(params TokenType[] endAt)
 		{

Pattern.g.cs

@@ -7,7 +7,7 @@
 
 namespace Finn.AST;
 
-public abstract record Pattern() {
+public abstract record Pattern(Token Start) {
 	public abstract TResult accept<TContext, TResult>(TContext context, IPatternVisitor<TContext, TResult> visitor);
 }
 public interface IPatternVisitor<TContext, TResult> {
@@ -16,28 +16,28 @@ public interface IPatternVisitor<TContext, TResult> {
 	TResult visitFieldPatternPattern(TContext context, FieldPattern pattern);
 	TResult visitRecordPatternPattern(TContext context, RecordPattern pattern);
 }
-public partial record SimplePattern(Token? Identifier) : Pattern()
+public partial record SimplePattern(Token? Identifier, Token Token) : Pattern(Token)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IPatternVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitSimplePatternPattern(context, this);
 	}
 }
-public partial record VariantPattern(Token Tag, Pattern? Argument) : Pattern()
+public partial record VariantPattern(Token Tag, Pattern? Argument, Token Backtick) : Pattern(Backtick)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IPatternVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitVariantPatternPattern(context, this);
 	}
 }
-public partial record FieldPattern(Token Name, Pattern? Pattern) : Pattern()
+public partial record FieldPattern(Token Name, Pattern? Pattern) : Pattern(Name)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IPatternVisitor<TContext, TResult> visitor)
 	{
 		return visitor.visitFieldPatternPattern(context, this);
 	}
 }
-public partial record RecordPattern(FieldPattern[] Fields, SimplePattern? Rest) : Pattern()
+public partial record RecordPattern(FieldPattern[] Fields, SimplePattern? Rest, Token LBrace) : Pattern(LBrace)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IPatternVisitor<TContext, TResult> visitor)
 	{

Program.cs

@@ -41,16 +41,16 @@ class Program
 	{
 		var scanner = new Scanner(src);
 		List<Token> tokens = scanner.scanTokens();
-		Console.WriteLine("TOKENS\n=======");
+		// Console.WriteLine("TOKENS\n=======");
-		foreach (var token in tokens)
+		// foreach (var token in tokens)
-		{
+		// {
-			Console.WriteLine(token);
+		// 	Console.WriteLine(token);
-		}
+		// }
 		Parser parser = new Parser(tokens);
 		Expr? expression = parser.parse();
-		Console.WriteLine("\nAST\n=======");
+		// Console.WriteLine("\nAST\n=======");
-		Console.WriteLine(expression);
+		// Console.WriteLine(expression);
-		Console.WriteLine();
+		// Console.WriteLine();
 
 		if (hadError)
 		{
@@ -84,17 +84,17 @@ class Program
 	{
 		if (token.Type == TokenType.EOF)
 		{
-			report(token.Position, " at end", message);
+			report(token.Start, " at end", message);
 		}
 		else
 		{
-			report(token.Position, " at '" + token.Lexeme + "'", message);
+			report(token.Start, " at '" + token.Lexeme + "'", message);
 		}
 	}
 
 	public static void runtimeError(RuntimeError err)
 	{
-		Console.Error.WriteLine($"{err.Message}\n[{err.Token.Position}]");
+		Console.Error.WriteLine($"{err.Message}\n[{err.Token.Start}]");
 		hadRuntimeError = true;
 	}
 
@@ -167,7 +167,7 @@ public record struct Position(int Offset, int Line, int Column)
 	}
 }
 
-public record Token(TokenType Type, String Lexeme, Object? Literal, Position Position);
+public record Token(TokenType Type, String Lexeme, Object? Literal, Position Start, Position End);
 
 class Scanner
 {
@@ -222,7 +222,7 @@ class Scanner
 
 	private void addToken(TokenType type, Object? literal)
 	{
-		tokens.Add(new Token(type, lexeme, literal, start));
+		tokens.Add(new Token(type, lexeme, literal, start, current));
 	}
 
 	private bool match(char expected)
@@ -409,7 +409,7 @@ class Scanner
 			scanToken();
 		}
 
-		tokens.Add(new Token(TokenType.EOF, "", null, current));
+		tokens.Add(new Token(TokenType.EOF, "", null, current, current));
 		return tokens;
 	}
 }

ast_classes.fsx

@@ -1,65 +1,93 @@
 type Field = { Type: string; Name: string }
-type Type = { Name: string; Fields: list<Field> }
+type Type = { Name: string; Fields: list<Field>; Start: string }
 
-let exprTypes =
+let exprTypes: Type list =
     [ { Name = "Sequence"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Right" } ] }
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Right" } ]
+        Start = "Left.Start" }
       { Name = "Binary"
         Fields =
           [ { Type = "Expr"; Name = "Left" }
             { Type = "Token"; Name = "Op" }
-            { Type = "Expr"; Name = "Right" } ] }
+            { Type = "Expr"; Name = "Right" } ]
+        Start = "Left.Start" }
       { Name = "Grouping"
-        Fields = [ { Type = "Expr"; Name = "Expression" } ] }
+        Fields = [ { Type = "Expr"; Name = "Expression" } ;{Type="Token";Name="LParen"}]
+        Start = "LParen"}
       { Name = "Literal"
         Fields =
           [ { Type = "System.Object"
-              Name = "Value" } ] }
+              Name = "Value" }; {Type="Token"; Name="Token"} ]
+        Start = "Token"}
       { Name = "Unary"
-        Fields = [ { Type = "Token"; Name = "Op" }; { Type = "Expr"; Name = "Right" } ] }
+        Fields = [ { Type = "Token"; Name = "Op" }; { Type = "Expr"; Name = "Right" } ]
+        Start = "Op" }
       { Name = "If"
         Fields =
           [ { Type = "Expr"; Name = "Condition" }
             { Type = "Expr"; Name = "Then" }
-            { Type = "Expr"; Name = "Else" } ] }
+            { Type = "Expr"; Name = "Else" }
+            { Type = "Token"; Name = "IfToken" } ]
+        Start = "IfToken" }
       { Name = "Variable"
-        Fields = [ { Type = "Token"; Name = "Value" } ] }
+        Fields = [ { Type = "Token"; Name = "Value" } ]
+        Start = "Value" }
       { Name = "List"
-        Fields = [ { Type = "Expr[]"; Name = "Elements" } ] }
+        Fields = [ { Type = "Expr[]"; Name = "Elements" }; {Type="Token"; Name = "LBracket"} ]
+        Start = "LBracket" }
       { Name = "Variant"
-        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Expr?"; Name = "Argument" } ] }
+        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Expr?"; Name = "Argument" }; {Type="Token";Name="Backtick"}]
+        Start = "Backtick" }
       { Name = "Record"
         Fields =
           [ { Type = "Field[]"
               Name = "Extensions" }
-            { Type = "BaseRecord?"; Name = "Base" } ] }
+            { Type = "BaseRecord?"; Name = "Base" }
+            { Type = "Token"; Name= "LBrace"} ]
+        Start = "LBrace"}
       { Name = "Selector"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Token"; Name = "FieldName" } ] }
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Token"; Name = "FieldName" } ]
+        Start = "Left.Start" }
       { Name = "Indexer"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Index" } ] }
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Index" } ]
+        Start = "Left.Start" }
       { Name = "Call"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr?[]"; Name = "Arguments" } ] }
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr?[]"; Name = "Arguments" } ]
+        Start = "Left.Start" }
       { Name = "Let"
         Fields =
           [ { Type = "Binding[]"
               Name = "Bindings" }
-            { Type = "Expr"; Name = "Body" } ] }
+            { Type = "Expr"; Name = "Body" }
+            { Type = "Token"; Name = "LetToken"} ]
+        Start = "LetToken" }
       { Name = "When"
-        Fields = [ { Type = "Expr"; Name = "Head" }; { Type = "VarBinding[]"; Name = "Cases" } ] } ]
+        Fields =
+          [ { Type = "Expr"; Name = "Head" }
+            { Type = "VarBinding[]"
+              Name = "Cases" }
+            { Type = "Token"; Name = "WhenToken"} ]
+        Start = "WhenToken" } ]
 
 let patternTypes =
     [ { Name = "SimplePattern"
-        Fields = [ { Type = "Token?"; Name = "Identifier" } ] }
+        Fields = [ { Type = "Token?"; Name = "Identifier" }; {Type = "Token"; Name = "Token"}]
+        Start = "Token" }
       { Name = "VariantPattern"
-        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Pattern?"; Name = "Argument" } ] }
+        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Pattern?"; Name = "Argument" }; {Type="Token"; Name = "Backtick"} ]
+        Start = "Backtick" }
       { Name = "FieldPattern"
-        Fields = [ { Type = "Token"; Name = "Name" }; { Type = "Pattern?"; Name = "Pattern" } ] }
+        Fields = [ { Type = "Token"; Name = "Name" }; { Type = "Pattern?"; Name = "Pattern" } ]
+        Start = "Name" }
       { Name = "RecordPattern"
         Fields =
           [ { Type = "FieldPattern[]"
               Name = "Fields" }
             { Type = "SimplePattern?"
-              Name = "Rest" } ] } ]
+              Name = "Rest" }
+            { Type = "Token"
+              Name = "LBrace"} ]
+        Start = "LBrace" } ]
 
 let visitorMethod baseName t = $"visit{t.Name}{baseName}"
 
@@ -82,7 +110,7 @@ let renderType (sw: System.IO.StreamWriter) baseName t =
     List.iteri writeField t.Fields
 
     sw.Write
-        $") : {baseName}()
+        $") : {baseName}({t.Start})
 {{\n"
 
     sw.Write
@@ -107,7 +135,7 @@ let renderAST outputDir baseName types =
 
 namespace Finn.AST;
 
-public abstract record {baseName}() {{
+public abstract record {baseName}(Token Start) {{
 \tpublic abstract TResult accept<TContext, TResult>(TContext context, I{baseName}Visitor<TContext, TResult> visitor);
 }}\n"
 

strawman4.txt

@@ -1,3 +1,6 @@
+# v5
+# Added implicit (dynamic) bindings
+
 # v4
 # Moved type annotations into bindings.
 # Added def keyword for top-level bindings.
@@ -30,6 +33,8 @@ type alias tree 'a = recurse 'T in <leaf, branch: {left: 'T, value: 'a, right: '
 type alias maybe 'a = <some: 'a, nothing>
 type alias unit = {}
 
+implicit print(s: string) -> unit 
+
 # Type aliases should appear in compiler messages.
 # binary_tree takes a comparison function and returns a record (like a JS
 # object) as a module for handling binary trees of values of the same type
@@ -38,7 +43,7 @@ def binary_tree(compare: fn('a) -> 'a) -> {
 	empty: tree 'a,
 	insert: fn(tree 'a, 'a) -> tree 'a,
 	find: fn(tree 'a, 'a) -> maybe 'a,
-	@"func we don't care about": fn() -> unit
+	@"func we don't care about": fn() -> |print| unit
 } =
 	# A let expression can bind multiple names. Functions have access to all names within the expression,
 	# while other values only have access to names bound before them.
@@ -65,7 +70,7 @@ def binary_tree(compare: fn('a) -> 'a) -> {
 	in {empty, insert, find, @"func we don't care about"}
 
 # Prints "`some(2)".
-def do_tree_things() =
+def do_tree_things() -> |print| unit =
 	# Assume that int_compare is in the stdlib or something.
 	let {insert, empty, find | @"stuff we don't need"} = binary_tree(int_compare)
 	# I invented this fancy partial-application syntax with _, and then found
@@ -76,7 +81,7 @@ def do_tree_things() =
 		@"stuff we don't need"
 
 # Prints "`some(2)\nignore me".
-def main() = do_tree_things().@"func we don't care about"()
+def main() = with print = native_print in do_tree_things().@"func we don't care about"()
 
 def annotate(note: 'a, obj: {|'b}) -> {note: 'a | 'b} =
 	{ note | obj }